1. Field of the Invention
The present invention relates to a method of dynamically allocating frequencies for use in a digital, mobile, portable telephone system achieving communications using a multicarrier time division multiple access (TDMA), and in particular, to a dynamic frequency allocation method for use with a plurality of asynchronous digital, mobile, portable telephone systems capable of preventing radio inferences due to interferences between the respective systems.
2. Description of the Related Art
Recently, the mobile communications have been developed to an amazing extent; especially, the mobile portable telephone systems have been put to various usages.
In the present stage of technology of the mobile portable telephone, there has been commonly adopted a frequency division multiple access (FDMA) system in which a radio communication is achieved by use of a particular frequency in a one-to-one type procedure between a mobile station and a base station wired to a fixed telephone network.
In the mobile portable telephone systems employing the FDMA procedure, carrier selection ranges assigned to the respective systems are different from each other such that each system is allowed to exclusively utilize a unique carrier. In consequence, considerations need not be given to the possibility of common uses of an identical carrier between two or more systems.
On the other hand, there has been employed a multiplex communication system other than the FDMA communication system above, namely, a TDMA multiplex communication system in which a carrier having a preset frequency is subdivided with respect to time for communications.
The TDMA multiplex communication system has already been used in a communication system conducting communications between fixed stations. In particular, there has been adopted in many cases a multicarrier TDMA communication system using a plurality of carriers in which each carrier is subdivided into a plurality of time slots. When a communication is achieved between a fixed central station and a fixed terminal station, a channel search is carried out through the channels each defined by a combination of a carrier frequency and a time slot to select therefrom a free channel not being used for a communication between other central and terminal stations, thereby achieving the communication.
In the conventional multiplex communication of the multicarrier TDMA procedure between the fixed central station and the fixed terminal, carrier selection ranges allocated to the respective communication systems between the fixed stations are different from each other. Namely, the communication is accomplished therebetween in accordance with a slaved or subordinate synchronization in which a fixed terminal station establishes synchronization with respect to a fixed central station in a subordinate manner.
As a result, in an identical communication system between fixed terminal stations, there does not occur an interference between the terminal stations; moreover, the carrier selection ranges vary between the communication systems, thereby unnecessitating the users to consider the possibility of the common use of an identical carrier between the systems.
Incidentally, in the mobile portable telephone communication and an automobile telephone communication employing digitalized or digital signals, adoption of the multicarrier TDMA communication system has been discussed for the following reasons. Namely, in each communication system, the base station is allowed to access a plurality of mobile stations to decrease the number of base stations to be installed, thereby reducing the system cost; furthermore, the communication system is capable of coping with an increase in the number of subscribers without installing additional base stations.
In the TDMA procedure in the mobile portable telephone system and the automobile telephone system employing the digital signals, the base station and the mobile station respectively correspond to the central station and the terminal station disposed in the conventional TDMA communication system between the fixed stations.
However, since the mobile portable telephone system and the automobile telephone system employing the digital signals are characterized in that the respective terminal stations are mobile, namely, are not fixed, it is difficult to allocate a fixed carrier frequency to be used by each pair of a base station and a mobile station.
In order to overcome this difficulty, there is used a dynamic channel allocation. That is, the carrier used by the pair of a base station and a mobile station is not fixed such that a pair of a mobile station and a base station ordinarily in a state waiting for a fixed carrier searches a plurality of carriers for a free channel or an available channel at initiation of a communication, thereby conducting the communication through the channel.
Under this condition, in the case of the automobile telephone system, since a connection object to which the central station i.e. the automobile telephone base station of the TDMA communication system is to be connected is an automobile telephone network, when the base station of each automobile telephone system establishes a subordinate synchronization with respect to synchronization of the automobile telephone network, the synchronization between the respective base stations, namely, between the automobile telephone systems can be kept retained. This consequently guarantees the base stations to allocate the respective time slots thus attained through the time division to the associated terminal stations without any fears of interferences between the automobile telephone systems.
Unlike the automobile telephone system, the connection object of the mobile portable telephone system of the former case is in general a public telephone or pay station network (PSTN), a private branch exchange (PBX), or the like. Consequently, when achieving the subordinate synchronization with respect to these networks, it is quite difficult to acquire a synchronization timing signal from the network side. As a result, in each mobile portable telephone system, the base station is disadvantageously required to independently generate a transmission clock signal.
In this regard, conventionally, according to the carrier selection method adopted in the mobile portable telephone communication, each telephone system selects a carrier from a carrier range uniquely assigned thereto, and hence it is not necessary for each base station to comprehensively control the carrier ranges related to all base stations. That is, there has not been disposed a device for the comprehensive carrier control.
In consequence, when the TDMA procedure is utilized in the mobile portable telephone communication of the prior art, since any base station of a telephone system cannot recognize a carrier selected by a base station of another system, there may possibly occur a chance where an identical carrier is selected by a plurality of base stations in the mobile portable telephone communication.
In this situation, however, the base station of each system independently generates the transmission clock signal as described above. Consequently, when the transmission clock frequencies are not synchronized with each other between the base stations of the respective telephone systems and the widths of the transmission clock signal are different from each other between a plurality of base stations selecting an identical carrier, the synchronization timing of the time slot varies between the respective systems.
FIGS. 5 and 6 show the signal operation states in the situations above.
First, FIG. 5 is a signal timing chart showing with bipolar pulse images a state of a communication initiation in which base stations KS1 and KS2 of two different telephone systems respectively start communications with mobile stations PS1 and PS2 by using different time slots of an identical carrier. In this chart, the upper and lower portions respectively show communication states developed in the base stations KS1 and KS2, respectively.
In the diagram of FIG. 5, a division of each axis of abscissa represents a time slot, and the numerals over and below the axis designate numbers assigned to the associated time slots. Moreover, time slots shown over the axis are respectively used to transmit signals from the base stations KS1 and KS2 to the mobile stations PS1 and PS2, respectively. In addition, time slots shown beneath the axis are respectively employed to send signals from the mobile stations PS1 and PS2 to the base stations KS1 and KS2, respectively.
Furthermore, time slots each enclosed in a frame are respectively acquired by a pair of the base station KS1 and the mobile station PS1 and a pair of the base station KS2 and the mobile station PS2 in an independent manner for communications.
In the chart of FIG. 5, the base station KS1 initiates a communication in a first time slot of a carrier; whereas, the base station KS2 starts a communication in a third time slot of the same carrier.
However, these two base stations KS1 and KS2 operate in an asynchronous manner with respect to each other and the transmission clocks respectively thereof are generated in a separate fashion. Consequently, an interval of time slots generated by the base station KS1 is not necessarily equal to that of the time slots produced by the base station KS2.
When the transmission clock interval of the transmission clocks created by the base station KS1 is different from that of the transmission clocks generated by the base station KS2, a difference between the transmission timings for the time slot of the base station KS1 and that of the base station KS2 varies. For example, when the time slot length of the pair of the base station KS1 and the mobile station PS1 is less than that of the pair of the base station KS2 and the mobile station PS2, the third time slot used by the base station KS2 is shifted, with a lapse of time, backward relative to the first time slot utilized by the base station KS1.
As a result, the interval of about at least 2.5 slots between the first slot employed by the base station KS1 and the third time slot used by the base station KS2 at the initiation of the communication shown in FIG. 5 is changed or decreased with a lapse of time to the state of FIG. 6. Namely, the interval between the slots is gradually decreased and hence signals transmitted from the respective stations may cause interferences with each other in some cases.
More concretely, on confirming that these time slots do not interfere with each other at the starting point of the communication, the base stations KS1 and KS2 acquire these time slots for communication. FIG. 5 shows relationships between the time slots at the initiation of the communication. The state of these time slots is then changed, in this case, the distance therebetween is reduced as shown in FIG. 6. This inevitably leads to a problem of an occurrence of a collision which is an overlap in time between time slots used by the base stations and hence deteriorates the speech or telephone communication quality.